Modern vehicles usually include various driver assistance systems which, in certain driving situations, automatically intervene in the vehicle operation in order to stabilize the vehicle or to otherwise assist the driver. Among these are, in particular, vehicle dynamics control systems such as ESP (electronic stability program), but also “driver-assistance systems” such as ACC (adaptive cruise control), as well as extra-value functions like, for example, APB (automatic parking brake) or a hill-hold function (HHC). These systems usually drive a hydraulic pump which builds up a desired setpoint brake pressure at a wheel. As a rule, the hydraulic pump is actuated in time-controlled manner. The driving time is defined by a model stored in the software.
Because of tolerances in the overall brake system, especially during the first phase of the pressure buildup, it is only conditionally possible to exactly reach the desired target pressure solely through the duration of the pump operating time. The brake pressure actually reached may therefore deviate relatively sharply from the target pressure. That is why known brake systems include a pressure-limiting valve, which is used to limit the pressure prevailing in the brake circuit to a predefined value. When the brake pressure generated by the hydraulic pump reaches the threshold value set at the valve, the valve becomes overloaded, and therefore the pressure is limited. In this case, the pressure threshold value is controlled via the electric valve current.
However, in systems having a pressure-limiting valve, it is problematic that the setting of the valve is also faulty. The pressure threshold value set can therefore deviate from the target pressure. In known systems, such a deviation from the target pressure is corrected by the superordinate vehicle controller. However, in the case of deceleration in the partial-braking range, this control process can lead to reactions (e.g., deceleration fluctuations, noises) noticeable to the driver, thereby impairing the control comfort.